Plant primary growth is mainly driven by an enlargement of the cells, which occurs through the irreversible yielding of the primary cell wall to turgor pressure inside the cell. Although cell division is required to produce new cells, the growth results from the expansion of these cells, not simply from their division. Cellulose microfibrils, which are embedded in a matrix of hemicellulose and lignin in the wall, are the main determinants of tensile strength (Appenzeller et al., Cellulose 11:287-299 (2004)). A cell usually expands along the axis that is perpendicular to the orientation of the microfibrils. For example, radial deposition of microfibrils favors cell expansion along the longitudinal axis.
Secondary wall differs from primary wall in that it is richer in cellulose and lignin and its deposition commences toward the end of cell expansion. Modulation of primary cell wall synthesis has applications in altering growth rate and size (stature) of a plant whereas that of secondary wall can be useful in improving biomass accumulation and tissue strength (Appenzeller et al., Cellulose 11:287-299 (2004)).
Cellulose in general is the major wall constituent in mature plant cells forming vegetative tissues. The paracrystalline structure of cellulose that results from energy minimization by the formation of inter- and intra-chain hydrogen bonds makes it mechanically one of the strongest organic molecule known on density basis. It is natural then that cellulose is the primary determinant of strength in structural tissues.
Plant mechanical strength is one of the most important agronomic traits. Plant mutants that are defective in stem strength have been isolated and characterized. Barley brittle culm (bc) mutants were first described based on the physical properties of the culms, which have an 80% reduction in the amount of cellulose and a twofold decrease in breaking strength compared with those of wildtype plants (Kokubo et al., Plant Physiol. 97:509-514 (1991)). Rice brittle culm1 (bc1) mutants show a reduction in cell wall thickness and cellulose content (Qian et al., Chi. Sci. Bull. 46:2082-2085 (2001)). Li et al. described the identification of rice BRITTLE CULM1 (BC1), a gene that encodes a COBRA-like protein (The Plant Cell 15(9):2020-2031 (2003)). Their findings indicated that BC1 functions in regulating the biosynthesis of secondary cell walls to provide the main mechanical strength for rice plants.
The stalks of maize brittle stalk 2 (bk2) mutant exhibit a dramatically reduced mechanical strength compared to their wild type counterparts (Langham, M N L 14:21-22 (1940)). Maize bk2 mutants have stalk and leaves that are very brittle and break easily. The main chemical constituent deficient in the mutant stalk is cellulose. Therefore, stalk mechanical strength appears to be dependent primarily on the amount of cellulose in a unit length of the stalk below the ear.
Furthermore, genes encoding cellulose synthase catalytic subunits (CesA) have been implicated in cell wall synthesis and are represented by a large family in plants. Ten genes were identified in Arabidopsis after complete genome sequencing and twelve genes have been isolated from maize by EST sequencing (U.S. Pat. Nos. 6,803,498 and 6,930,225). Three of the CesA genes from each Arabidopsis and maize have been reported to make secondary wall whereas the rest apparently make primary wall (Taylor et al., Proc. Natl. Acad. Sci. U.S.A. 100:1450-1455 (2003)). Mutations in three of the CesA genes from Arabidopsis resulted in collapsed xylem and reduced mechanical strength of the stem-like peduncle. When related CesA genes from rice were mutated the culms became brittle, indicating the role of these genes in secondary wall formation. In each case, reduced mechanical strength was correlated with diminished cellulose content.
In general, mutations in the CesA genes involved in primary wall formation cause severe phenotypic alterations whereas those in secondary wall-forming genes do not alter the visual phenotype as much as they affect mechanical strength (Appenzeller et al., Cellulose 11:287-299 (2004)).
As insufficient stalk strength is a major problem in corn breeding, it is desirable to provide compositions and methods for manipulating cellulose concentration in the cell wall and thereby alter plant stalk strength and/or quality for improved standability or silage quality.